Many current wireless receivers use multiple antennas to reduce multipath and interference on signal reception. The utilization of multiple receive antennas can provide a diversity gain.
A technique for low-complexity antenna diversity is described in an article entitled “Low-Complexity Antenna Diversity Receivers for Mobile Wireless Applications,” by Zhang, C. N., Ling, C. C., International Journal on Wireless Personal Communications, pp. 65-8. The authors show the viability of combining diversity antenna signals using front-end analog circuits to achieve significant diversity gain in comparison to conventional diversity techniques that require duplicate signal paths and demodulators. The technique described in the article provides hardware savings by eliminating one of the demodulators. Furthermore, since each antenna is receiving the same desired channel, the need for duplicate local oscillator is eliminated. Likewise, channel selection filters, amplifiers and data conversion hardware can be shared.
Patent application Ser. No. 12/247,908, filed Oct. 8, 2008, entitled “Low-Complexity Diversity Using Coarse FFT and Subband-Wise Combining,” the content of which is incorporated herein by reference in their entirety, disclose a diversity combining receiver which combines the diversity signals prior to baseband and demodulator processing, as shown in FIG. 1.
FIG. 1 is a block diagram of a three-antenna diversity receiver 100 described in “Low-Complexity Diversity Using Coarse FFT and Subband-Wise Combining”, patent application Ser. No. 12/247,908, filed Oct. 8, 2008. Although receiver 100 is shown as including three paths (channels), namely paths 140, 145 and 150, it is understood that a diversity receiver, in accordance with the above referenced patent application, may have any number of paths. Path 140 is shown as including an amplifier 102-1, a frequency conversion module 104-1, and an analog-to-digital converter 306-1. Path 145 is shown as including an amplifier 102-2, a frequency conversion module 104-2, and an analog-to-digital converter 106-2. Path 150 is shown as including an amplifier 102-3, a frequency conversion module 104-3, and an analog-to-digital converter 106-3.
Each amplifier 102-i, where i is an index ranging from 1 to 3, is configured to receive and amplify an input signal received from an associated antenna 130-i. In one embodiment, each amplifier 102-i may be a Low Noise Amplifier (LNA). In another embodiment, each amplifier 302i may be a LNA having a variable gain. Amplifier 102-i may be configured as a single-stage or multi-stage amplifier.
The output signal of amplifier 102-i is shown as being applied to an associated frequency conversion module 104-i. Frequency conversion modules 104-i are shown as being mixers in exemplary embodiment. Each mixer 104-i is configured to frequency down-convert the received signal using the oscillating signal generated by local oscillator 148. The signal whose frequency is down converted by mixer 104-i is converted to a digital signal by analog-to-digital (ADC) converter 106-i. FFT module 108-i transforms the time-domain digitized signal into a frequency domain using 2m points, as described further below.
Assume that the bandwidth of the signals ASi supplied by ADC 106i is BW. For a particular wireless channel, the frequency selectivity has a coherence bandwidth CBW, which is the frequency bandwidth across which the channel can be approximated as a flat channel. CBW is inversely proportional to the delay spread of the channel. The delay spread can, in turn, be extracted from the channel's impulse response. Parameter K which is defined by rounding the ratio (BW/CBW) provides a guideline for the number of points the FFT 108-i may require, by choosing the smallest m such that 2m>=K.
The bin (or subband) output signals FSi of the associated FFT modules 108-i may be combined after cophasing or combined using MRC, hence referred to herein to as subband MRC. The SNR of each subband may be estimated using any one of a number of conventional techniques to implement MRC. For example, relative subband amplitude combined with gain information available in the analog front end may be used to provide subband-wise signal strength information. The resulting signal CS1 is transformed back to time domain by IFFT module 312. The output of IFFT module 112 is filtered by lowpass filters 114, 116, and amplified by variable gain stage 118. The output of variable gain stage 118 is applied to a demodulator 120.
As shown in FIG. 1, bin-wise combiner 110 combines the output signals of FFT modules 108-1, 108-2 and 108-3 to generate signal CS. The combined signal CS is, in turn, applied to IFFT 112 that generates signal DS by transforming signal CS from frequency domain to time domain. The parameter m, which is the number of points used in FFT modules 108-i, may be selected independently from the type of signal modulation being received. In an exemplary embodiment, an OFDM system may have 4096 subbands and, during demodulation, requires a 4096-point FFT. In accordance with the present invention, a significantly smaller FFT module may be used to perform the diversity processing, thus greatly reducing complexity and power consumption. The present invention may be equally applied to non-OFDM signals (e.g. single-carrier or CDMA signals) with relatively the same degree of effectiveness.